A fixing device is adapted to be mounted on a plate that is formed with a mounting hole, and includes an upper fixing unit, a lower fixing unit, a string member and a first fastener. The upper fixing unit has two upper string through holes and a fastener through hole. The lower fixing unit has a first fastener engaging hole. The string member includes two penetration sections respectively extending through the upper string through holes and being connected to the lower fixing unit. When said string member is pulled, the first fastener engaging hole and the fastener through hole are aligned with each other. The first fastener extends through the fastener through hole to be engaged with the first fastener engaging hole.